1. Field of the Invention
The invention relates to a freewheeling diode device for a switchable device component including a commutator branch.
2. Brief Description of the Background of the Invention Including Prior Art
A so-called freewheeling diode is used in switching circuits with inductive load such as described, for example, in the German Printed Patent document P-4,105,155.6 of the applicants. The requirements relative to the respective freewheeling diodes increase with the quality of switching device components. In addition to a blocking voltage loadability of, for example, more than 1200 volts, to a low forward voltage drop and to a small reverse current, there is required a soft recovery behavior, i.e. a soft reverse current behavior, wherein reference is made to a property related to the dynamic behavior of the diode, as described in the German Printed Patent document DE-3,823,795-A1. This soft reverse current behavior of the diode concerns the emptying of a plasma of charge carriers from the semiconductor body during switching from the conducting phase into the blocking phase In case of a soft recovery behavior, the reverse current decays slowly. Since the induced surge voltage corresponds to this decay behavior, only a small voltage is generated in case of a soft switching diode, whereas in contrast high voltages can be induced in connection with a diode with a hard or snappy switching behavior. The characterizing parameter for the recited recovery behavior is the so-called softness factor, as is defined in the above-recited reference, German Printed Patent document DE-3,823,795-A1.
A freewheeling diode is essentially a rectifier diode connected across an inductive load to carry a current resulting from the energy stored in the inductance when no power is being supplied by the source to the load and until all the energy in the inductance has been dissipated or until the next voltage
In connection with devices of fast-recovery power diodes of conventional construction, the recovery behavior is influenced by doping and dimension of the high-resistive center zone of the three-layer structure of the semiconductor body.
A fast-recovery diode is essentially a diode in which very little carrier storage occurs and which may therefore be used to give an ultrahigh speed of operation.
Depending on the presetting of the reverse blocking voltage loadability, the maximum doping concentration can amount to, for example, 1.times.10.sup.14 cm.sup.3 for 1000 volts and to 7.times.10.sup.13 cm.sup.3 for 2000 volts with a coordinated minimum thickness of the center zone of 80 microns or, respectively, 180 microns. To make a soft recovery diode there exists a general rule that the n.sup.- layer is to be produced in such doping and in such thickness, that the charge carrier zone does not reach the n.sup.+ zone in case of a presence of maximum allowable operating voltages.
However, the recovery behavior cannot be optimized in a desired way, based solely on the dimensioning of the width of the center zone by way of a predetermination of doping concentration and blocking capability, as has been shown by investigations. The requirements to the softness factor w, depending on the application case and depending on the application of the presently offered switchable semiconductor device elements, are not achievable with such dimensioning of a fast-recovery power diode.
It is further known to employ an irradiation with protons and an irradiation with helium nuclei for setting the local carrier lifetimes and thus for influencing the switching behavior in the sense of a changing of the recovery behavior. This advantage of this method is the substantial apparatus expenditure and effort required in processing times.
A further constructive possibility includes a subdivision of a p anode zone into two zones, of which the inner zone exhibits a surface concentration of from 1.times.10.sup.15 to 2.times.10.sup.16 cm.sup.3 and where the outer zone, having a doping level concentration larger than 3.times.10.sup.17 and a penetration depth less than 2 micrometers, serves only for ohmic contacting. Such structure is taught, for example, in German Patent Application Laid Open DE-OS 3,633,161. This structure furnishes an improved recovery behavior based on a substantial lowering of the charge carrier concentration on the p side. The structure taught in German Patent Application Laid Open DE-OS 3,633,161, however, is associated with a substantially reduced surge current in addition to higher production costs and expenditures.
In addition, it is possible to subdivide this p zone into several sub-regions, of which one exhibits a higher penetration depth, as well as to import n.sup.+ doped islands into the p zone. Again, the disadvantage associated with such structure resides in an increased production expenditure based on a requirement of masked diffusion.
In connection with known construction shapes of fast power diodes, the recovery behavior is influenced, for example, by doping of the emitter, by special emitter structures, or by proton irradiation. It has been found, however, that these steps for the improvement of the soft recovery behavior are obtained at the expense of the forward voltage drop. This causes high on stage losses and high switching losses. In particular, the switching losses on one of the limit for the use of the known freewheeling diodes with soft recovery behavior.
The dynamic loadability of freewheeling diodes has been considered in lecture notes for the 20th Colloquium Semiconductor Power Device Elements and Material Quality of Silicon in a lecture entitled "Dynamic Loadability of Freewheeling Diodes" by U. Scheuermann, J. Lutz, R. Spiegl of the Semikron company in Nuremberg, Germany. This paper shows how the dynamic behavior of various freewheeling diodes can be determined and that certain problems arise in the dynamic behavior when freewheeling diodes are commutated.
The journal article by S. D. Brotherton et al., entitled "A Comparison of the Performance of Gold and Platinum Killed Power Diodes" published in Solid-State Electronics, Vol. 25, No. 2, pages 119-125 (1982) shows the influence of the distribution of deep impurities such as gold upon the detailed switching characteristics.
The journal article by Yu. V. Bulgakov et al., in the publication Soviet Physics Semiconductors, Vol. 18, Issue 9, September 1984, pages 1009-1010 brings an article entitled "Investigation of the Profile of Recombination Parameters of Proton-irradiated Silicon". The article describes a method for determining the profile of the diffusion length of minority carriers in silicon.
The journal article by E. E. Velmre, in the Soviet publication Electrical Engineering, Vol. 55, No. 3, pages 47-51 (1984), entitled "Influence of the Lifetime Distribution of Electrons and Holes on the Reverse Recovery Process of p.sup.+ -n-n.sup.+ Diodes" shows the results of a study of the reverse recovery process of diffused diodes.
The German Printed Patent Document DE-3,631,136-A1 to Peter Voss teaches a diode with a soft recovery behavior. The reference teaches the production of such a diode.
The German Patent DE-2,506,102-C3 to Hansjochen Benda et al., teaches a semiconductor rectifier. The reference is concerned to provide a structure which dampens high-frequency oscillations in a semiconductor rectifier.
The German Patent DE-3,008,034-C2 to Yoshihito Amemiya et al. teaches an electrode for semiconductor device elements. The teaching of the reference is directed to reducing the voltage drop in forward direction and the recovery time of a diode.